Ok so listen up Rosalinas because I'm only going to say all of this once
Greninja screws up Rosalina's recovery. Hydro Pump can very easily make Rosalina shoot herself over the stage and Greninja has more than enough mobility to catch up to her and punish while she's still vulnerable which can lead to a really easy kill.
Rosalina is an easy target. Tall and floaty is the kind of character Greninja loves to fight (just ask Mewtwo) and so Rosalina is very easy to hit with things such as retreating F-Air that can leave him safe and possibly causing damage to Rosalina and Luma. Speaking of which:
Luma is not hard for Greninja to dispose. As mentioned before, Dash Attack and Jab 3 send Luma flying easily and after Luma suffers a bit of damage, F-Air and F-Smash (to a lesser extent Shadow Sneak as well) are also very good at disposing of her. Plus, we can pelt Luma with shurikens to build up damage against her and the only answer Rosa has to that is GP which is not exactly threatening as Rosalina can't punish us with it and if nothing else it leaves Luma without protection for Greninja's other attacks.
Rosalina dies really quickly. A sweetspot Up-Smash from Greninja can kill her as soon as 80% and considering we have kill confirms into Up-Smash, Rosalina is at danger of dying at around 70%. That's not counting with her potentially being caught by F-Smash or Shadow Sneak at the edge or being smacked with B-Air or Shadow Sneak offstage
Respect the Dash Grab. Greninja's dash grab is really good due to it's great range and so he can really punish Rosalina for her mistake with it and quickly throwing her before Luma can interrupt.
As you can see Greninja does have quite a few things going for him in this MU, especially when it comes to edgeguarding as he can really ruin Rosalina's day offstage.
Neutral: Rosalina has an advantage as long as Luma is still alive due to her taking the shurikens for Rosa and our N-Air not being as useful while she's still alive due to the hitlag hitting Luma with it causes and the fact that it's not very good for killing her, as well as that excellent jab of hers. Greninja has to take out Luma in order to win here, as without Luma to protect her, Rosalina is really vulnerable during GP to deal with shurikens and Greninja's amazing mobility can make running away for Rosalina while without Luma really difficult. While she can still fight back without Luma, Greninja can really wreck Rosalina when she's by herself due to being much more mobile than her and having his great dash grab alongside his amazing advantaged state.
Speaking of which, Greninja's
Advantage state is likely one of the best in the game due to his high mobility, good range, excellent aerials for juggling and ability to easily catch landings with dash grab and dash attack. Rosalina's advantaged state is not to be underestimated either as her Up-Air is definitely very silly and can kill us really quickly if we're not careful, however once Luma is gone her advantaged state deflates considerably.
Disadvantage: Rosalina's D-Air can prevent Greninja from juggling her too much, with or without Luma and is probably her best asset to reset back to neutral. Greninja meanwhile can escape Rosalina's juggles with Hydro Pump which lets him get back to the ground quickly and considering Rosalina's floatiness and low fall speed, might even let him reverse the situation immediatelly to Rosalina likely still being above him once he's back to the ground.
Edgeguarding: Greninja wins here by a longshot, his recovery is very versatile with both Shadow Sneak and Hydro Pump covering so much distance that his recovery is near ungimpable without making a hard read and allows for too many mix-ups for Rosalina to be able to consistently edgeguard him, while Rosalina's recovery is very straight-forward and gets completely ruined by Hydro Pump and leaves her wide open for punishment.
Conclusion: The MU is even, a solid 50:50. Rosalina can keep Greninja away from her as long as Luma is still alive and good Rosalinas will keep Luma safe at all costs, making it difficult for the frog to get in and start doing his stuff, it's not impossible though and Greninja can certainly hold his own against Rosalina even with Luma still alive. It's when Luma is dead that things change dramatically and Rosalina is extremely vulnerable to Greninja's shenanigans during this time and any good ninja will take advantage of her vulnerability to turn the tables here, of course, Luma doesn't really stay away for too long.
It's the edgeguarding that makes this MU even because Greninja can severely mess Rosalina up when she's trying to recover and can very easily turns the table against her if she's caught by Hydro Pump while trying to use Launch Star. There's literally no risk in using Hydro Pump against Launch Star and it brings Greninja tons of reward for something so simple to do.